Conventionally, nitride semiconductor light-emitting elements in which a light-emitting layer that emits light by recombination of carriers (electrons and holes) is composed of a GaN-based semiconductor or an InGaN-based semiconductor have been widely used. Meanwhile, nitride semiconductor ultraviolet light-emitting elements having a shorter emission wavelength than the nitride semiconductor light-emitting elements mentioned above and including a light-emitting layer composed of an AlGaN-based semiconductor are proposed in, for example, Patent Document 1 and the like, but still have not been widely used.
This is because in the nitride semiconductor ultraviolet light-emitting elements including the light-emitting layer composed of the AlGaN-based semiconductor, as the peak emission wavelength is shortened (as the Al composition ratio of the light-emitting layer is increased and the Ga composition ratio thereof is reduced), the light emission efficiency is reduced. The light emission efficiency of a semiconductor light-emitting element is represented as quantum efficiency that is the ratio of converting injected electrons into photons. The ratio when focusing on photons generated inside a light-emitting element is referred to as internal quantum efficiency whereas the rate when focusing on photons discharged outside the light-emitting element is referred to as external quantum efficiency.
Such a problem is described with reference to the drawings. FIGS. 5 and 6 are graphs of the relationship between the peak emission wavelength and the external quantum efficiency of nitride semiconductor light-emitting elements. FIG. 5 is a graph described in Non-Patent Document 1 and obtained by collecting data reported in academic papers and the like by various companies and research institutions. FIG. 6 is a graph obtained by measuring samples made by the inventors of the present application in the past. The horizontal axis of each of the graphs shown in FIGS. 5 and 6 represents the peak emission wavelength whereas the vertical axis thereof represents the external quantum efficiency. In FIG. 5, for the convenience of description of the drawings, a curve that is not a precise approximate curve but represents the overall trend of points and a broken line that indicates a peak emission wavelength of 285 nm are added to the graph described in Non-Patent Document 1.
As shown in FIGS. 5 and 6, in nitride semiconductor ultraviolet light-emitting elements having a peak emission wavelength of 285 nm or shorter, as the peak emission wavelength is shortened, the external quantum efficiency is sharply reduced. This shows that the nitride semiconductor ultraviolet light-emitting elements having a peak emission wavelength of 285 nm or shorter have their specific problems that do not occur in nitride semiconductor light-emitting elements having a peak emission wavelength longer than 285 nm. In addition, these problems occur in the nitride semiconductor ultraviolet light-emitting elements having a peak emission wavelength of 285 nm or shorter without exception, and even in recent years, have not yet been solved.